Existing communications systems typically rely for transmission over long distances upon the use of nominally single mode optical fibres which carry optical signals and provide transmission of signal data at 10 Gb/sec or more over distances of the order of 100 km. Although such fibres are nominally single mode, propagation of optical signals is generally characterised in such fibres by two orthogonally polarised HE.sub.11 modes for which slightly different group velocities exist in the presence of birefringence.
For a given span of optical fibre, the difference in transmission time for these modes is termed polarisation mode dispersion.
For the given span of optical fibre, it is possible to define a pair or orthogonal principal polarisation states such that an optical pulse launched into the fibre in only one of the principal polarisation states will be received at the other end of the fibre without polarisation mode dispersion being evident, the principal polarisation states therefore representing the fast and slow axis modes of propagation. In practical systems however, it is difficult to control the launch state to always correspond to one of the principal polarisation states so that an optical signal typically comprises the sum of fast and slow mode components.
Environmental factors affecting the optical fibre produce variation over time in the birefringence effects causing polarisation mode dispersion and the resulting dispersion is observed to vary relatively slowly for fibres in buried cables and more quickly for fibres contained in overhead cables.
It is known from U.S. Pat. No. 5473457, Ono, to analyze a received optical signal in a manner which permits the principal states of polarisation to be determined and the received pulse separated into fast and slow mode components, the fast mode component then being subject to a compensating delay by means of transmission of both components through a polarisation maintaining optical fibre of predetermined length and high polarisation dispersion to provide a differential delay. This technique however has the disadvantage of making available only a fixed amount of compensation and therefore does not allow variable compensation of polarisation mode dispersion suitable for a practical communications system. A further disadvantage is that a delay element providing optical delay by transmission via a fibre will typically require a relatively long length of fibre in the range 10 to 100 meters.
It is known from WO 97/50185 to compensate for polarisation mode dispersion by splitting the received optical signal at the receiver into two polarisation states and to apply switched delays of different length to the separated components, thereby providing a variable delay. A disadvantage of this system is that the delay is not continuously and smoothly variable and also requires a relatively complex optical switching configuration.
The inventor of the present invention has previously disclosed in U.S. Pat. No. 4953939 the use of a chirped Bragg grating reflector in combination with a directional coupler to introduce a delay which is wavelength dependent because the periodicity of the Bragg grating varies with position along the fibre so that different wavelengths are reflected from different positions along the fibre. The inventor has also disclosed in U.S. Pat. No. 5602949 a technique for manufacturing a suitable chirped fibre by producing strain in a non-chirped fibre for this purpose. GB-A-2316761 discloses a method of directly writing a chirped grating into a fibre for the purpose of compensating chromatic dispersion.
There remains a need to provide an improved method of providing a continuously variable optical delay and for compensating for polarisation mode dispersion in optical fibres.